Various forming techniques are being used for fabrication of metal and ceramic parts. In the manufacturing of many metal and ceramic articles, a “green” body is formed of powder, binder, and possibly other ingredients. The green body is then typically fired to remove the binder and sinter to form the dense metal or ceramic articles. One of the desired methods to make green bodies is an injection molding process.
In injection molding, a mixture of metal and/or ceramic powder and binder is injected into a mold corresponding to the desired green body shape. Typically the mixture is heated to lower its viscosity before molding. The mixture is allowed to harden in the mold, and the mold is then opened to remove the green body from the mold cavity. The green body is then fired to remove the binder and sinter the part to near theoretical density.
More specifically, waxes are commonly employed as binders because they fulfill the rheological requirements of high fluidity at moderately elevated temperatures (150-220° C.) and substantial rigidity at temperature below 15° C. Wax formulations normally comprise between about 10 to 20% wax by weight of the formula. During the firing process, wax is initially removed from the green body. This initial step of the firing process may have an adverse influence on the quality of the post-molding sintered parts. This step requires exclusive equipment and very long binder burnout time to avoid the development of creaks in the part.
As an alternative to the aforesaid injection molding process, the use of agaroid as binder in the manufacture of parts from metals or ceramic powders has been disclosed (U.S. Pat. No. 4,734,237). This binder system resolve the binder burnout difficulty associated with the wax binder systems. However, higher cost associated with agaroid binder system, which significantly reduces the profit margin of the final product. Also, the low gelling temperature (35-39° C.) of agaroid binder system prolongs the molding cycling time, which adversely effects the production rate. The feedstock material is compounded using twin screw machine to mix the binder, metal powder and water to produce pellets. The pellets are transferred to the hopper of an injection molding machine for molding.
This invention provides novel molding compositions useful in forming metal and/or ceramic parts, which not only allow for the production of complex shapes and reduction of the firing times for such parts, but also reduced the cost of the binder by 33% and the molding cycle time by 25%. Furthermore, this invention provides a Direct Compounding/Molding technique, which eliminates the use of twin screw machine for compounding feedstock. This technique will hereafter be referred to as “DCM.”
This invention also teaches a new aqueous binder system to form a thin or thick singles or multi layers of metal, ceramic or metal/ceramic composite coating on the green molded articles. The selected area or the whole green injection molded parts could be spray or brush coated with the specific ferrous, non-ferrous alloys, refractory metals or ceramic coating slurry made with this binder. This coating creates a dense layer after sintering.